N-(phosphonacetyl)-L-aspartate (PALA), a transition-state analog inhibitor of the reaction catalyzed by aspartate transcarbamylase is a promising addition to the group of antitumor drugs. The chemical and biological effects of PALA will be studied in relation to the growth and viability of normal and cancer cells as a basis for optimal therapeutic application. In vivo testing has shown PALA to be extremely effective against Lewis lung carcinoma and B16 melanoma, but little antitumor activity was found against L1210 and xenographs of human colon cancer in mice. Synthesis and testing of several new inhibitors of de novo pyrimidine nucleotide biosynthesis are proposed. Analogs of PALA are being prepared with the purpose of extending the range and efficiency of antitumor activity by obtaining improved potency. Potential transition state analog inhibitors of pyrimidine-specific carbamyl-P synthetase are also being synthesized. This presents a better target by being considerably less active than aspartate transcarbamylase and because it is the site of feedback regulation of the entire pathway.